The control of interior noise is a formidable problem. The sources of noise are many and varied; for example, in modern turbo-fan commercial aircraft, the turbulent boundary layer (TBL), engine noise and auxiliary equipment noise can all contribute to the interior noise. The primary source of interior noise is the turbulent boundary layer that forms around the exterior surface of the aircraft in flight, with the additional contribution of noise from the engines towards the rear of the aircraft. The random pressures generated in the TBL cause motion of the aircraft sidewalls, which then radiate sound into the interior. This sound then propagates through the walls and ceiling and into the interior of the aircraft. Reflected sound from the interior surfaces builds up a semi-reverberant noise field.
Current conventional noise control treatments are effective at moderate to high frequencies and so the residual noise spectrum is dominated by lower frequency noise centered around 300 Hz. At these frequencies, the use of conventional treatments is hampered by severe weight and volume constraints. Active noise control offers the potential of effective performance without a significant weight penalty.
Several studies, using a variety of approaches, have considered the application of active noise control for aircraft interior noise. A few systems have been fully developed, are available commercially and are in regular service, specifically for the control of tonal noise in the interior of propeller aircraft. Systems proposed for broadband interior noise include control of fuselage vibration, activated absorptive treatments, activated panels and enhanced double wall panel systems. Each of these systems has advantageous features and implementation drawbacks.